This invention relates generally to A.C. motors and more particularly to starting control circuits for single phase induction motors.
A.C. single phase induction motors require a starting method to rotate the magnetic field of the windings to generate sufficient torque to start rotation of the rotor. Such motors have an auxiliary or start winding in addition to a main winding. During start-up of the motor both the main and auxiliary windings are energized to bring the motor up to speed. Once operating speed is reached the torque supplied by the auxiliary winding is no longer needed and for optimum operational efficiency of the motor it is necessary to effectively disconnect the auxiliary winding.
It is known to place a triggerable solid state switch such as a thyristor in series with the auxiliary winding across an A.C. power source. Conduction between the main terminals of the switch is initiated by applying a triggering current to the gate terminal and is terminated when the triggering current is reduced below a predetermined level. Examples of such controls are shown and described in U.S. Pat. Nos. 3,544,869 and 3,643,142 assigned to the assignee of the present invention. In the former patent, a thermistor is interconnected with the gate terminal of the device for controlling the flow of triggering current thereto. The thermistor reduces the triggering current below the predetermined triggering level when it is heated to a predetermined threshold temperature. A heater thermally coupled to the thermistor is connectable across the auxiliary winding so that it is energized to heat the thermistor concurrently with the energization of the auxiliary winding. The solid state switch is triggered to supply current to the auxiliary winding upon energization of the motor and the heater thereby heats the thermistor. After a predetermined delay to reach the threshold temperature, the thermistor reduces the triggering current below the predetermined triggering level of the switch and thereby reduces the current in the auxiliary winding. After this occurs, the heater remains energized by the back E.M.F. voltage induced in the auxiliary winding when the motor is operating thereby maintaining the thermistor above its threshold temperature.
In the U.S. Pat. No. 3,643,142 patent, a PTC thermistor is connected to the gate terminal of a solid state switch for controlling the triggering signals applied thereto. The PTC thermistor is connected to the power supply and is selected so that it self-heats and undergoes a transition into a high-resistance mode of operation without the necessity for externally applied heat and remains in a relatively stable temperature condition upon the establishment of its high-resistance mode of operation.
Although the above noted apparatus can be effective on starting an induction motor and thereafter disconnecting the auxiliary winding, once the auxiliary winding is deenergized it remains de-energized until power to the motor is removed. When power is removed, the PTC thermistor cools to its low-resistance state thereby enabling restarting of the motor.
There are instances where it would be desirable to effectively monitor the main winding current and to reenergize the auxiliary winding to help the motor overcome an overload condition should one occur, however, that is not possible with the above noted motor starting circuits.